1. Field of the Invention
The present invention relates to a process for efficiently isolating, or separating, a xylene isomer(s) and/or ethylbenzene useful as a solvent and a starting material for chemical syntheses, and to an agent for use in isolation, or separation, of a xylene isomer(s) and/or ethylbenzene. Starting materials usable in the process of the present invention include mixtures of at least two xylene isomers selected from among o-xylene, m-xylene and p-xylene; and mixtures of ethylbenzene and at least one xylene isomer selected from among those mentioned above. These mixtures may further contain a small amount of impurities.
2. Prior Art
Xylenes are widely used as solvents and starting materials for chemical syntheses of synthetic resins, synthetic fibers, and the like. Among xylene isomers, p-xylene in particular is in great demand.
On the other hand, ethylbenzene is used as a starting material of styrene monomer, and the like.
Individual xylene isomers (o-isomer, m-isomer and p-isomer) are usually obtained from the so-called mixed xylene (o-isomer: about 20%, m-isomer: about 40%, p-isomer: about 15%, ethylbenzene: about 15%, other compounds such as styrene: small in amount). o-Xylene and ethylbenzene can be separated from the mixed xylene and isolated from each other by precision distillation wherein a difference in boiling point therebetween is utilized. m-Xylene and p-xylene can hardly be isolated from each other by distillation because the boiling points thereof are extremely close to each other. The common processes for isolating m-xylene and p-xylene from each other include a low-temperature processing method, an adsorption method, and an MGCC method (developed by Mitsubushi Gas Chemical Company, Inc.). The low-temperature processing method, in which a large difference in melting point between m-isomer and p-isomer is utilized, is not commonly used these days because it involves a drawback of processing at as low a temperature as -70.degree. C. and a large consumption of energy used to lower the temperature. The adsorption method, in which a zeolite molecular sieve is used as an adsorbent (UOP ISOMAR AND PAREX PROCESS developed by Universal Oil Products Co., U.S.A.), is said to provide a high yield of p-xylene having a purity of 99% or higher even when the mixed xylene is passed only once through the molecular sieve. The MGCC method, in which selective complexing of m-xylene with a mixture of hydrogen fluoride and boron trifluoride is utilized, enables pure m-xylene to be recovered in a substantial yield of 100%. Thus, the adsorption method and the MGCC method are both excellent ones. However, the former disadvantageously requires a large amount of a solvent for use in a mobile phase, while the latter involves a drawback of handling hydrogen fluoride, which is an intractable substance.
In view of the above, an object of the present invention is to provide a novel and economical process for highly selectively isolating, or separating, a xylene isomer(s) and/or ethylbenzene by using a substituted .alpha.-cyclodextrin, developed with the aim of improving the water solubility of .alpha.-cyclodextrin. Another object of the present invention is to provide an agent usable for isolation, or separation, of a xylene isomer(s) and/or ethylbenzene.